The invention pertains to the field of micromachined integrated valves and, more particularly, to the field of low leak rate, normally closed valves for industrial and ultra clean applications. Semiconductor processing applications are examples of ultra clean applications.
Normally open, micromachined integrated valves are known in the prior art. Examples of various embodiments of such valves are given in U.S. Pat. Nos. 4,821,997 and 4,824,073 and 4,943,032 and 4,966,646 by Mark Zdeblick, assigned to Stanford University, the disclosures of which are hereby incorporated by reference. Such valves generally include a three die structure which uses the top two dies to form a sealed cavity having a fluid with a boiling point selected for the application to which the valve will be put, said fluid trapped therein and having one wall formed as a thin, flexible silicon membrane. A resistive element is also in the cavity on the top die and has electrical connections to the outside world by which current may be passed through the resistor. The bottom die has a valve seat formed therein and input and output flow channels which are coupled across the valve seat. When current is passed through the resistor, the trapped fluid is heated and expands thereby causing the flexible membrane to flex far enough to come into contact with the valve seat and cut off fluid flow from the input channel to the output channel.
Normally open valves are suitable for some applications, but in other applications normally closed valves are needed wherein the deenergized state (trapped fluid not heated) results in no flow from the input channel to the output channel. Normally closed valves can be micromachined on substrates having similar structures and using similar manufacturing techniques as the normally open integrated electric-to-fluidic valves. An example of such a normally closed valve is the Fluistor.TM. Microvalve (NC-105) manufactured by Redwood Microsystems, Inc. of Menlo Park, Calif. In this valve, the flexible membrane is coupled at one point by a pedestal to the bottom die in which the output channel is formed (the "bottom die" refers to the lowermost layer in which the input or output channel is formed, but the general term "die", as that term is used herein, both with reference to the prior art structures and the invention, refers to the separate layers of the structures shown in the figures regardless of the material of which they are made). The middle die in which the flexible membrane is formed also has formed into the silicon thereof a flat valve seat. Under this valve seat there is formed an output (or input) channel through the bottom die. When the valve is de-energized, the flat valve seat sits on top of the channel through the bottom die and cuts off flow therethrough. When the flexible membrane flexes as the trapped fluid is heated, the flexing movement is constrained at the position of the pedestal and is converted into a torque which causes the middle and top dies (the top die seals the fluid in the cavity) to pivot upward around the pedestal as pivot point. This raises the valve seat off the channel formed through the bottom die thereby opening the valve. The fluid flow is typically over the top of the upper two dies, through a separation between the second and third die and out a channel formed in the third die. The reverse flow pattern is less preferred, but acceptable.
The current Fluister.TM. (trademark of Redwood Microsystems, Inc.) valve works well for noncorrosive or nonflammable fluids to be controlled and where leak rates on the order of approximately 1.times.10.sup.-4 cc-Atm/sec or more of Helium are good enough. However, in ultra clean processing and some industrial and medical applications, corrosive gases or fluids need to be controlled, and in some of these applications leak rates of less than 1.times.10.sup.-6 cc-Atm/sec of Helium or better must be obtained. Further, in many applications, it is important that the integrated valve be constructed so as to not add any undesirable materials (contaminants) to the fluid stream being controlled. These contaminants may derive from or originate from the material of the valve or its attachment material. These contaminants would interfere with or degrade the semiconductor processing or add undesired material to the semiconductor devices being manufactured. Examples of such undesired materials include things such as metals, more specifically, alkali metals, or other substances such as organics. Further, because many of the fluids that must be handled in a manufacturing processing environment are corrosive and would eventually destroy most containment materials, very careful selection of materials in the wetted area of the fluid stream of an integrated silicon valve must be used to prevent contamination of the fluid stream and to minimize any corrosive effects.
A prior art integrated valve which is not chemically compatible with most processing environments is the NC105 manufactured by Redwood MicroSystems, Inc. of Menlo Park, Calif. This valve does not have a sealing ring around a chemically compatible wetted area to prevent fabrication fluids from reaching parts of the valve which are not "chemically compatible" as that term is defined herein and typically used.
Accordingly, a need has arisen for a normally closed integrated, microminiature valve which can control corrosive fluids for long periods of time without failing, and which, in some embodiments, has a low leak rate across the valve seat and to the outside world.